Bath for plating gold on titanium metal

ABSTRACT

A plating bath for plating gold on zirconium or titanium metal by the chemical displacement method comprising a soluble gold salt, a salt for attacking the metal oxide dioxide and a stabilizing agent. A cleaning bath containing fluorides and a method of use of both cleaning bath and plating bath are described. The plated articles are suitable for use as storage battery grids.

United States Patent 11 1 Nordblom [45] June 24, 1975 BATH FOR PLATING GOLD ON TITANIUM METAL {75] Inventor: George F. Nordblom, Yardley, Pa. [73] Assignee: ESB Incorporated, Philadelphia, Pa.

[22] Filed: Dec. 12, 1973 [21] Appl. No.: 424,173

[52] US. Cl. 106/1; 117/130 E; 204/29 [51] Int. Cl. C23c 3/00 [58] Field of Search 106/1; 204/29; 117/130 E;

[56] References Cited UNITED STATES PATENTS 3/1958 Missel et a1. 106/1 X 3/1964 Pokras et a1. 8/ 1972 Kendall 204/29 X OTHER PUBLICATIONS Condensed Chemical Dictionary; 7th Edition, Reinhold Publishing Corp.; pgs. 56, 57; Sci. Lib., GD 5 C5.

Primary ExaminerLewis T. Jacobs Attorney, Agent, or FirmRobert H. Robinson, Esq.;

Wm. Wharton Smith, Anthony J. Rossi, Esq.

[ 5 7 ABSTRACT BATH FOR PLATING GOLD ON TITANIUM METAL BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to solutions used for the gold plating of metals. In particular, it relates to a solution for plating gold on titanium metal without the use of electric current. Such plated metal is suitable for storage battery grids.

2. Description of the Prior Art The light weight and strength of titanium metal makes it attractive as a high performance structural material. It has the further feature of being highly resistant to most forms of electrolytic corrosion. Studies have shown that the corrosion resistance is due to the formation of a highly inert and very tenacious surface layer of titanium dioxide. Unfortunately, titanium dioxide is a non conductor of electricity. The oxide forms when titanium metal is anodized in an electrolyte environment. When cathodized, the oxide layer breaks down and becomes conductive with the net result that a wet titanium surface becomes in fact a rectifier.

Because of this, when titanium is used in applications where it will carry electric currents and where electrolytes are present, special means must be provided to allow for the entry and exit of electrical current therefrom. One such means is to plate a portion or all of the surface of the titanium part to prevent the formation of the oxide layer at the plated area and to provide a direct metallic path from the surface of the part to its interior. A number of plating materials have been suggested. One desirable plating material is gold. Because of the high cost of gold, it is desirable to have the gold plate as thin as possible as well as being continuous and firmly adhered to the titanium base.

Most plating of metals is done by electrodeposition processes. Electroplating will provide a plating layer of any desired thickness. The plating is not always a complete covering and minute cracks, pinholes or other imperfections may occur in the plated surface. A second method of plating in limited commercial use is known as electroless plating. In this method, the surface to be plated is rendered catalytic to the deposition of metal from a plating solution and the metal plates out. In true electroless plating the deposited metal is autocatalytic so that the thickness of the metallic coating is not limited. Electroless plating has found a place in industry because it is not limited by the current paths of the electroplating process. There are limitations to the process, particularly in the need for more or less con tinuous surveilance of the plating bath.

A third method of plating, and one that is used extensively in the precious metal and jewelry industry, is the chemical displacement method. When a base metal is placed in a solution of a noble metal, ions of the base metal go into solution and at the same time an electrochemical equivalent of the nobel metal ions deposit on the surface of the base metal. When the entire surface of the base metal is coated, the action ceases. The coating so formed is thin compared to most electroplated coatings, being on the order of molecules thick rather than fractions of a millimeter in thickness. Further, the coating is very thorough without cracks, pinholes, etc.

In the plating of titanium, it is necessary that the coating of titanium dioxide be completely removed to provide a complete and continuous surface. Plating baths for electroplating gold onto a titanium surface are known. A number of baths for the electroless plating of gold are known. Other metals are known to have a refractory oxide coating, particularly the group [Vb metals, zirconium and hafnium. Tungstic acid and other tungsten compounds have been used in gold plating baths as grain refiners in electroless gold plating solutions.

SUMMARY OF THE INVENTION A plating bath for gold plating of titanium metal by chemical displacement includes: monovalent gold ions, citrate ions for complexing monovalent gold, fluoride ions for decomposing the titanium dioxide surface layer, tungstate ions for preserving the bath and hydrogen ions to place the bath in an acidic state. Preferred concentration of the several ions include:

monovalent gold up to 25 gms per liter citrate about 50 gms per liter fluoride 10 to 20 gms per liter hydrogen ion to give pH of 5-6 The bath is also useful for plating titanium alloys, zirconium and other metals.

It is seen that this bath is characterized by a minimum number of components, each component having a speciftc part in the action of the bath. The bath is convenient and easy to use. The cleaned titanium part is placed in the solution until the surface is coated and then the part is removed. The bath can be stored and does not deteriorate. It can be used until the gold therein is practically completely plated out.

The gold plate produced by this bath is continuous, smooth and without faults. The deposit of gold tends to stop off at about 2 to 5 milligrams per square inch of surface. It is firmly adherent to the titanium substrate and provides an ideal means for electrical contact to a titanium part.

DESCRIPTION OF THE PREFERRED EMBODIMENT The plating bath of the present invention includes the following chemical entities:

a. a soluble gold complexing agent containing monovalent gold;

b. a material for chemically removing the metallic oxide; and

c. a stablizing agent to prevent the decomposition of the solution with age.

There are few soluble monovalent gold compounds. One available soluble compound is potassium gold cyanide. Cyanide solutions, however, are very hazardous to use. A preferred material for plating work is gold citrate. This is a non-hazardous material. It complexes the gold in monovalent form and is suitable for use in the acid bath of the present invention. It is not used in an exact ratio to the gold, about 50 gms/liter is desirable.

A preferred preparation for gold citrate comprises reacting potassium gold cyanide with ammonium citrate at an elevated temperature until the evolution of hydrocyanic acid fumes have ceased. The small amount of cyanide remaining is not considered dangerously toxic and may in fact improve the quality of the plating. Since hydrocyanic acid is an extremely toxic material, this preparation must be carried out in an enclosed space and means must be provided for the further decomposition and removal of the poison.

A preferred ion for attacking titanium dioxide is the fluoride ion in acid solution. This can be added to the gold citrate solution in the form of ammonium fluoride with ammonium acid fluoride, although other fluorides such as sodium or potassium fluorides with proper control of the acidity may also be selected. The acid fluoride is used to set and control the acidity of the solution to a pH of about 5-6. Fluoride solutions attack glass, therefore, when the preparation of the gold citrate is complete, the solution is cooled and transferred to a plastic container for the addition of the fluoride containing materials. These are added and dissolved.

It is important that the ratio of ammonium fluoride to ammonium acid fluoride be held at about 1 to 0.8 in order to get the desired acidity. In developing the solution, an all ammonium fluoride addition was tried with poor results. Ammonium acid fluoride was added a bit at a time until the ratio of l to 0.8 was reached, at which point best results were obtained. Further additions of the acid fluoride reduced the effectiveness of the bath.

It was found that the solution described above produced a very desirable thin gold coat on titanium metal. However, in using this solution, it was observed that the solution tended to decompose. After some use, the solution turned a purplish color and the gold precipitated out. As a complete surprise, it was found by trial that the addition of a small quantity of sodium tungstate prevented the decomposition of the used solution.

Tungstic acid is known to be a grain refiner in gold plating baths. in the development of the bath of the invention, titanium was placed in a hot bath of gold citrate and fluorides. There was a rapid plating out of gold, but the coverage was poor. Sodium tungstate was added to improve the plating quality 55.4g/liter. The plating was slowed down and took minutes for coverage, the coverage was poor and the color was poor. The plating solution was not providing sufficient removal of the surface oxides. To correct this, a deoxidizing bath containing ammonium fluoride and ammonium acid fluoride was developed.

A solution of ammonium fluoride and ammonium acid fluoride was prepared. Best results were obtained when the bath contained about 14 g per liter of ammonium fluoride and about 21 gms per liter of ammonium acid fluoride. Also, it was found desirable to hold the ratio of ammonium fluoride to ammonium acid fluoride at about 2:3.

Titanium grids were placed in the hot fluoride solution and immediately placed in a gold plating bath without tungstate ion at room temperature. A good adherent deposit of gold occurred within 2 minutes. It was decided to add 5 gms/liter tungstate to the solution. The plating time remained the same and the high quality of the plate was maintained. lOg/liter tungstate was tried with the same results. However, when 25g/liter was tried, the plating time was extended to minutes. It was then observed that certain of the solutions prepared without the tungstate addition had decomposed after use. The solutions turned a bluish color and gold deposited. Further observations disclosed that solutions containing tungstate did not decompose with age Thus, it was found that the addition of sodium tungstate to the plating bath within the range of 5-l0 gms/liter, although not necessary as a grain refiner or as an inhibitor, appeared to be very desirable as a stabilizer and preservative for the bath.

In the preparation of the preferred solution, sodium tungstate is added along with the fluorides and dissolved in the solution. The fully compounded solution should be stored in a plastic bottle.

The bath is suitable for use with other metals which have a refractory oxide surface coating. Thus, it can be used for plating titanium alloys, and zirconium and its alloys.

During the transfer of the deoxidized part from the deoxidizing bath to the plating bath, it is important that the cleaned metal surface is not exposed to atmospheric oxygen or other oxygen source. Therefore, the part is moved as quickly as possible from the first bath to the second without rinsing and without allowing the fluoride solution on the surface to run off. There is some carry-over of chemicals. However, the addition of fluoride to the plating bath is not harmful and in fact tends to make up for the drag-out of fluoride from the plating bath.

In using a displacement type bath, it must be understood that the strengths of the components therein are in a state of constant change. In the present bath, the gold content decreases as gold is plated out on the substrate. In place of gold, substrate ions titanium for example will be found in the solution. There is no exact top limit for the concentration of gold ions. In practice however, a comparatively low gold content is desirable so as to reduce the drag-out losses or loss of gold solution with removal of plated parts from the bath. Thus, in generalities, gold plating baths much stronger than about lO-30 gms gold per liter are not used.

As the gold of the present bath is used up, the plating speed is reduced. This puts a practical limit on the low end of the concentration; namely, about f; to 1 gm of gold per liter of solution.

The bath may be used hot or cold. For pure titanium and certain titanium alloys, excellent coating and coating speed will be obtained with a room temperature bath. For more refractory titanium alloys as well as for zirconium, the bath must be heated up to 90C. for best results.

A preferred cleaning process for preparing metals for plating with the bath of the invention comprises steps 1 to 3 of the following:

I. removal of grease and organic material by immersion in a hot alkaline cleaner and then rinsed off;

2. dip in a solution of mixed ammonium fluoride and ammonium acid fluoride;

3. remove the part from fluoride bath and while still wet with solution so as to protect the surface from oxidation, immerse in the plating bath;

4. remove from the plating bath when the entire surface is coated usually 2-3 minutes, then wash and dry;

5. to obtain maximum adhesion, the part can be heat treated preferably in an inert atmosphere from 1-2 hours at 300400F.

EXAMPLE 1 To 500 ccs of distilled water, 50 gms dibasic ammonium citrate and 16 gms potassium gold cyanide (4l% -old) are added. The solution is heated to boiling and "old there A. hour or longer to remove all but traces of liydragcn cyanide. Note that this operation must be carried out under a suitable hood for safety. The solution is cooled and placed in a plastic beaker. Then 7.5 gms of ammonium fluoride, 6 gms ammonium acid fluoride and gms sodium tungstate are added and dissolved. The solution is then brought up to 1 liter total volume.

A second solution is prepared by dissolving 14 gms ammonium fluoride and 2l gms ammonium acid fluoride in 1 liter of distilled water.

A storage battery grid made from pure titanium sheet is washed in a hot alkaline cleaner solution. It is then immersed for 1% minutes in the second bath. The part is then removed from the cleaning bath and immediately placed in the plating bath at room temperature for 2 minutes. It is removed, washed and dried. The grid is then lead plated by conventional means, pasted with a lead oxide-sulfuric acid positive or negative paste and allowed to dry. The plate so prepared may then be used in the construction of a light weight lead acid storage battery cell.

EXAMPLE 2 Titanium alloy parts were machined to a desired finished shape. Four alloys were tested:

Alloy a c d Aluminum 6% 8% 6% Vanadium 4 1 Molybdel 6 l5 num Zirconium 4 5 Tin 2 Titanium remainder remainder remainder remainder adhered to the substrate.

It is to be understood that the above description applies to a single embodiment of the present invention and that other embodiments of the invention as well as other uses thereof will be apparent to the one skilled in the art. It is further to be noted that the method and baths of this disclosure are operable with other metals substrates than those specifically mentionedv Having fully described my invention, I hereby claim:

1. An aqueous bath for plating gold on metal by displacement which comprises:

a. monovalent gold ion, the concentration of gold ion being between the limits of about 25 gms per liter and about I gm per liter;

b. citrate ion, the concentration of citrate ion being about 50 gms per liter;

c. ammonium fluoride, the concentration of the ammonium fluoride being about 7 /2 gms per liter;

d. ammonium acid fluoride, the concentration of the ammonium acid fluoride being about 6 gms per liter; and

e. tungstic acid, the concentration of the tungstic acid being about 10 gms per liter.

2. An aqueous bath for plating gold on metal by displacement, the metal being selected from the group which consists of titanium, zirconium and hafnium which comprises:

a. monovalent gold ion, the concentration of gold ion being between the limits of about 25 gms per liter and about 1 gm per liter;

b. citrate ion, the concentration of citrate ion being about 50 gms per liter;

c. ammonium fluoride, the concentration of the ammonium fluoride being about 7 gms per liter;

d. ammonium acid fluoride, the concentration of the ammonium acid fluoride being about 6 gms per liter; and

e. tungstic acid, the concentration of the tungstic acid being about l0 gms per liter. 

1. AN AQUEOUS BATH FOR PLATING GOLD ON METAL BY DISPLACE MENT WHICH COMPRISES: A. MONOVALENT GOLD ION, THE CONCENTRATION OF GOLD ION BEING BETWEEN THE LIMITS OF ABOUT 25 GMS PER LITER AND ABOUT 1 GM PER LITER; B. CITRATE ION, THE CONCENTRATION OF CITRATE ION BEING ABOUT 50 GMS PER LITER; C. AMMONIUM FLUORIDE, THE CONCENTRATION OF THE AMMONIUM FLUORIDE BEING ABOUT 7 1/2 GMS PER LITER; D. AMMONIUM ACID FLUORIDE, THE CONCENTRATION OF THE AMMONIUM ACID FLUORIDE BEING ABOUT 6 GMS PER LITER; AND E. TUNGSTIC ACID, THE CONCENTRATION OF THE TUNGSTIC ACID BEING ABOUT 10 GMS PER LITER.
 2. An aqueous bath for plating gold on metal by displacement, the metal being selected from the group which consists of titanium, zirconium and hafnium which comprises: a. monovalent gold ion, the concentration of gold ion being between the limits of about 25 gms per liter and about 1 gm per liter; b. citrate ion, the concentration of citrate ion being about 50 gms per liter; c. ammonium fluoride, the concentration of the ammonium fluoride being about 7 1/2 gms per liter; d. ammonium acid fluoride, the concentration of the ammonium acid fluoride being about 6 gms per liter; and e. tungstic acid, the concentration of the tungstic acid being about 10 gms per liter. 